Alkoxysilane-functional polyurethanes which cure via silane polycondensation have long been known. A review article on this topic can be found e.g. in “Adhesives Age” April 1995, pages 30 ff (authors: Ta-Min Feng, B. A. Waldmann). Single-component polyurethanes of this type, which contain terminal alkoxysilane groups and which cure under the effect of moisture, are increasingly being used as flexible coating, sealing and adhesive compositions in the building trade and in the automotive industry.
These alkoxysilane-functional polyurethanes can be produced in accordance with U.S. Pat. Nos. 3,627,722 or 3,632,557 by reacting, for example, polyether polyols with an excess of polyisocyanate to form an NCO-containing prepolymer, which is then further reacted with an amino-functional alkoxysilane. Another way of producing alkoxysilane-functional polyurethanes consists, according to the teaching of EP-A 0 070 475, in capping hydroxy-functional polyurethane prepolymers with isocyanate-functional alkoxysilanes. According to EP-A 0 372 561, long-chain polyethers which have not been pre-extended via prepolymerisation can also be used here. However, all these systems based on polyether polyols form flexible polymers with relatively low strength after curing, which are less suitable e.g. for structural bonding.
Polyester-based alkoxysilane-functional polyurethanes have also been described already. For example, EP-A 0 354 472 or WO2004/005420 describe silane-curing hot-melt adhesives which, although they can achieve considerable tensile strength, are by nature solids at ambient temperature. While it is true that EP-A 0 480 363 also describes a special polyester-based system in which acrylate components are also modified, however, a solvent is obviously needed in this case in order to be able to achieve the desired viscosity. A modern adhesive system should not contain any solvent, however.
The polyester polyol-based, silane-curing polyurethane described in U.S. Pat. No. 6,756,456 is referred to as being at least “flowable”. In this case, however, the isocyanate used is TMXDI, which is accessible only with difficulty and is therefore expensive. In addition, the possibly low viscosity is obviously achieved by a polyether-polyester block structure. The patent specification provides no information on the tensile strength of such a system. As already explained, however, silane-curing polyurethanes containing polyether are rather flexible and exhibit a lower tensile strength.